The global environmental conservation and effective use of energy have recently drawn attention in various fields from the viewpoints of global warming, depletion of fossil fuel, and prevention of environmental pollution, and the like. Studies are therefore conducted on various environmental techniques.
It is important for those environmental techniques to clarify in detail combustion structure of combustion phenomenon in each of an engine, a burner, and the like, and transitional behavior thereof. A measurement technique using a semiconductor laser absorption spectroscopy has recently been developed as means of measuring distributions of temperature and concentration in the combustion gas in chronological order and with quick response.
Generally, an absorption spectroscopy is a measuring method utilizing property of gas molecules that absorb an infrared light having a wavelength specific to its chemical species and dependence property of the absorption amount on temperature and concentration of the gas. The concentration and the temperature of the gas to be measured can be measured by determining the ratio (Iλ/Iλθ) of intensity of a transmitted light (Iλ) to intensity of an incident light (λθ) acquired when the incident light is transmitted through an absorbing medium (the gas to be measured) having an even light path length.
One of techniques for detecting the property (concentration and temperature) of the gas to be measured, utilizing the absorption spectroscopy using the semiconductor laser is disclosed in Patent Document 1 and the like.
Patent Document 1 discloses a gas detecting apparatus that couples laser lights from two laser diodes with each other, and applies the coupled laser light to a target gas to calculate the gas concentration based on a measurement light transmitted through the target gas. Specifically, one laser diode generates a laser light having a wavelength to be absorbed by the target gas, and the other laser diode generates a laser light having a wavelength not to be absorbed by the target gas. A modulation signal regulator circuit modulates the laser lights to have substantially the equal amplitude and have the opposite phase, and produces modulated lights La and Lb. An optical multiplexer multiplexes the modulated lights La and Lb with each other to produce a measurement light Ls, and outputs the measurement light Ls to atmosphere of the target gas through an optical fiber and a collimator lens. The measurement light passes through the atmosphere of the target gas and is received by a photoreceiver, and a modulated component of an output signal thereof is extracted by a synchronous demodulating circuit. An arithmetic circuit calculates the concentration of the gas from the extracted modulated component.
In a gas detecting apparatus disclosed in Patent Document 2, a laser diode generating a laser light having a wavelength λ1 to be absorbed by a target gas outputs a modulated light La, and the modulated light La enters an optical splitter to be divided therein into two lights including a modulated light La1 and a modulated light La2. A first optical multiplexer receives the one modulated light La1 of the two divided modulated lights, and a modulated light Lb that is output from a laser diode generating a laser light having a wavelength λ2 not to be absorbed by the target gas, with the modulated light Lb having an amplitude equal to that of the modulated light La and having a phase opposite to that of the modulated light La. The first optical multiplexer produces a first measurement light Ls1. A second optical multiplexer receives the other modulated light La2 of the two divided modulated lights from the modulated light La, and a modulated light Lc that is output from a laser diode generating a laser light having a wavelength λ3 not to be absorbed by the target gas, with the modulated light Lc having an amplitude equal to and a phase opposite to those of the modulated light La. The second optical multiplexer produces a second measurement light Ls2. The first measurement light Ls1 and the second measurement light Ls2 enter a third optical multiplexer to finally produce a measurement light Ls.
When the concentration of the target gas is zero, the component having the wavelength λ1 of the measurement light Ls, is not at all attenuated. Therefore the measurement light Ls having cancelled modulated components and a constant intensity enters an optical receiver. When the value of the concentration of the target gas is not zero, the component of the measurement light having the wavelength λ1 is attenuated corresponding to the concentration of the gas due to the absorption thereby. Therefore, in the output of the optical receiver, a modulated component appears corresponding to the difference between the component of the measurement light having the wavelength λ1 and the components of the light for measurement having the wavelength λ2 and the wavelength λ3. With the detecting apparatus of Patent Document 1, thereby, the stability of the zero point can be maintained and gas leakage detection can be conducted more precisely regardless of any presence or any absence of various noises such as partial masking during the detection.
Patent Document 3 discloses a method of splitting a laser light with a branching filter into a laser light for measurement and a reference laser light, transmitting the laser light for measurement through a gas, receiving the transmitted light with an optical receiver, and determining an absorption spectrum absorbed by a gas component in the gas from the optical intensity of the received laser light for measurement and the optical intensity of the reference laser light.
Patent Document 4 discloses a method which sets a first time period during which an absorption wavelength specific to a gas-like substance to be measured is modulated with a predetermined frequency, and a second time period during which a wavelength not equal to the specific absorption wavelength is modulated with the predetermined frequency when an oscillation wavelength of a laser light is modulated with a modulation signal at a predetermined frequency. The disclosed method determines an accurate concentration of the gas by subtracting an offset signal measured during the second time period from a gas concentration signal including an offset signal measured during the first time period.